Semiconductor compounds of the III-V materials and alloys thereof; such as the compounds gallium arsenide (GaAs) and indium phosphide (InP) and the alloy aluminum gallium arsenide (AlGaAs) have unusual optoelectronic properties which make them attractive for many applications; ranging from microwave devices to optoelectronic devices. Among these applications is the use of such materials to make devices for high-speed logic integrated circuits and for microwave integrated circuits. The Schottky-barrier gate metal-semiconductor-field-effect transistor (MESFET) is a typical device used in these integrated logic circuits.
The MESFET is a three terminal device consisting of a source, gate and drain. The source and drain terminals form low resistance contacts to a channel whose conduction is controlled by the depletion field of a Schottky-barrier gate. The conducting channel, which is placed on a semi-insulating (SI) substrate, may be formed either by ion implantation into the semi-insulating material, or by epitaxially growing the active layer on semi-insulating material.
A number of problems associate with MESFET devices and circuits are attributed to the SI substrate. Such problems include backgating (or sidegating), hysteresis in the dependence of the drain-source current I.sub.ds upon drain-source voltage V.sub.ds, light sensitivity, low output resistance R.sub.d, low source-drain breakdown voltage BV.sub.SD, and low output power gain at RF frequencies. Among these problems, backgating is the most significant for both digital and analog circuit applications.
In addition to these problems, increased subthreshold leakage current, threshold voltage shifts, and the inability to fully pinch-off the device for large V.sub.ds can occur as the gate length of MESFET's is reduced to submicron dimensions. Also, R.sub.d and BV.sub.SD are further decreased as the gate length is reduced. These problems are called short-channel effects and the characteristics of the layer underlying the active region can have a profound influence on them.
Backgating or sidegating is the change of I.sub.ds in a MESFET as a result of a voltage applied to the substrate or an adjacent, and nominally isolated, contact pad (sidegate). Even though the sidegate and MESFET may be physically separated, as by mesa etching, the interaction may still arise because the substrate is of finite resistivity and charge can accumulate at the interface between the active layer and the substrate. In response to changes in voltage on the substrate or adjacent devices, the substrate conducts enough current to modulate the interface space-charge region. When this interfacial depletion region widens into the active channel, the device current is reduced.
A buffer layer is often inserted between the active layer and the substrate to alleviate the problem of backgating. To reduce backgating and other substrate related effects, the buffer layer should provide an increase in bulk resistivity. A number of possible buffer layers have been suggested, including undoped GaAs, AlGaAs, and superlattice (GaAs/AlGaAs) buffers. Heretofore such buffer layers have met with only limited success.
Another device useful for high-speed circuits is the MISFET (metal-insulator-semiconductor-field-effect-transistor). This, like the MESFET is a three terminal device. In the MISFET an insulator layer is formed between the underlying active channel region and the gate. In III-V devices it has been difficult to develop an insulator which meets all the requirements for such a layer.